We recently reported that incorporating unsaturation into long, lipid-like aliphatic side chains grafted onto imidazolium cations dramatically reduces the temperatures at which ionic liquids (ILs) containing those ions liquefy relative to counterparts bearing saturated side chains of equal length.  However, these alkene-induced reductions in Tm are accompanied by an increase in the chemical fragility of the ionic liquid.  Like edible oils containing unsaturated lipids, the former are subject to slow oxidative cleavage of their alkyl side chains at the site of unsaturation. In order to minimize this effect, the thiol-ene reaction was utilized with different ene-functionalized ionic liquids by reacting them photochemically with alkylthiols, yielding lipidic C16, C18 and C20-based ILs, with a thioether linkage.  In addition, a series of fatty acids with similar chain lengths were prepared via the thiol-ene chemistry for comparison. Differential scanning calorimetry (DSC) was conducted to study the impact of the structural modifications to the IL melting points, based on the sulfur position in the thioether linkage.Several different imidazolium-based ionic liquids were employed as ene substrates for the study. Imidazolium-based cations with either N-alkyl or N-alkenyl chains were chosen to assess the effect, if any, of having the sulfur moiety at various chain lengths from the center of the cationic charge.  All of the imidazolium-based ILs contained either C16, C18 or C20 alkyl appendages, similar to that of a natural fatty acids, with a thioether linkage at various positions within the alkyl chain. The synthetic methodology will be discussed, as well as the impact of these structural modifications upon ionic liquid melting points and computational insights into their basis.

Sulfur as an unsaturation effect isostere in lipidic materials:  T<sub>m</sub> lowering in lipid-inspired ionic liquids and fatty acids

243rd National Meeting (2012)

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Sulfur as an unsaturation effect isostere in lipidic materials: Tm lowering in lipid-inspired ionic liquids and fatty acids

Sulfur as an unsaturation effect isostere in lipidic materials: T_m lowering in lipid-inspired ionic liquids and fatty acids